Myogenesis in vitro has been much studied as a model of cellular differentiation. The formation of multinucleated non-proliferating electrically active and contractile myotubes by fusion of undifferentiated, proliferating myoblasts is accompanied by the appearance of muscle specific proteins, such as creatine kinase and myosin, and by the arrest of DNA synthesis. By manipulating the environmental conditions of a cloned line of myoblasts we are able to study these morphological and biochemical events. In addition, we hope to elucidate the nature of the unknown cue(s) that signal the cell to proceed along the path of differentiation rather than the path of continued proliferation.